LONDON — Manchester United’s slide into mediocrity was emphasized when it was humbled by the Athenian side Olympiakos.

An artful goal from Alejandro Dominguez, followed by a beautifully crafted score from 30 yards by Joel Campbell, sank United, 2-0, on Tuesday night in Greece. A wretched miscue from Robin van Persie that landed somewhere in the crowd was United’s abject response.

The result asks more questions about the quality of England’s Premier League, where money is no object and the Champions League appears to be a playground of embarrassing mishaps. For Manchester City to lose last week at home against Barcelona and for Arsenal to succumb to Bayern Munich is one thing. But the Greek champion outsmarted United tactically and was clearly the more motivated side.

It says much about the performance that Dominguez looked to be the finest player on the field throughout the contest. The 32-year-old Argentine’s career meandered through the Argentine, Russian and Spanish leagues before he landed in Piraeus, Greece. Yet he ran at United and appeared to dazzle them with his footwork long before his improvised flick of a foot from 13 yards out brought the first goal.

The second scorer, Joel Campbell, is a 21-year-old Costa Rican winger looking for a home in the sport. He is under contract to Arsenal but has never played for the London team. Instead, Campbell has had almost as many moves as Dominguez, flitting like a butterfly in loan moves to France, Spain, and now Greece.

He wandered into a central striking role because Olympiakos had sold Kostas Mitroglou to Fulham and because Javier Saviola is injured. But Michel, the former Spanish international player who has found his niche as a coach in the Greek capital, persuaded Campbell and the rest that they could take United.

Michel saw the flaws of an English team far from settled after the departure last summer of its veteran manager, Alex Ferguson. The new man in charge, David Moyes, has spent half a season trying to find the “devil” in the so-called Red Devils of Manchester.

It almost seems as if there has been a personality transplant, a loss of self-confidence among star players, without their old mentor. So many of them failed, not just in Athens but throughout the months of the English season, that the only way United seems likely to qualify for next year’s Champions League is to win the trophy this time around.

Moyes effectively told the players that. But he looked and sounded like a manager who does not know how to rouse them, how to guide or how to instill in them the devilment to go out and impose their qualities.

When Campbell struck the second goal on Tuesday, it was as if history was being turned on its head. He ran with the ball at a static line of defenders. He did not just beat Rio Ferdinand; he used the United defender as a decoy, bending his shot with the left foot around Ferdinand so that Manchester goalie David De Gea did not see the ball until it was too late.

A fabulous shot, struck with grace by a young player with his career ahead of him. Campbell was best known before this for his attempt to fake a foul against the United States in World Cup qualifying last September.

FIFA later reprimanded Campbell for bad sportsmanship. Arsenal will note now that the young man in whom it saw promise is growing into his potential.

And United, which last weekend made Wayne Rooney the richest player in the English game, can only regroup and prepare to make right in the second leg next month.

All the excuses, of injuries and transition after one of the longest reigns of management in club history, do not hide the embarrassment. Olympiakos is, clearly, a team playing above the level of the Greek league it dominates. Its coach sensed that Tuesday was an opportune time to take advantage of the nervous state he perceived in a weakened opponent.

However, there are degrees of weakness. Borussia Dortmund has lost starter after starter this season to long-term injuries, defenders especially.

Where it previously jousted with Munich in the Bundesliga, it now has fallen 20 points behind. And it arrived in St. Petersburg on Tuesday bruised by a 3-0 loss at Hamburg.

Jürgen Klopp, the Dortmund coach, would not accept a repeat performance, or excuse it because of the injuries. He prepared his team to come flying out of the gate in western Russia, where Zenit appeared still trapped in the winter hibernation.

By scoring twice in the opening five minutes, Dortmund set its marker for the match. The scorers, Henrikh Mkhitaryan and Marco Reus, were the players who have shined throughout Dortmund’s difficult season.

The Russian side, led by the mercurial Brazilian heavyweight, Hulk, stirred twice. A television microphone caught Klopp exhorting Dortmund’s Greek defender Sokratis Papastathopoulos, in English, “Don’t make him angry.” The reference was to Hulk. The fear was that a provoked Brazilian might turn the tide. Well, twice the big man did break Dortmund’s defenses.

At 58 minutes, he was the instigator of a goal for Oleg Shatov. At 70 minutes, Hulk scored with fearsome power from the penalty spot.

But Robert Lewandowski scored within two minutes of the first Zenit goal, then struck again moments after Hulk’s penalty. Dortmund’s flesh wounds were overcome by imperishable spirit in a 4-2 win.

The German club sent a message to Manchester United: Winning is a state of mind, about ambition when your resources are challenged, far from home, in the Champions League.